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APPLIED PHYSICS LETTERS 88, 032106 2006
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Metallic conductivity and metalsemiconductor transition in Ga-doped
ZnO
V. Bhosle, A. Tiwari, and J. Narayan
Department of Materials Science and Engineering, North Carolina State University,
Raleigh, North Carolina 27695-7907
指導老師：林克默 博士
報告學生：郭俊廷
報告日期：2011.03.01
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Outline
•
•
•
•
Introduction
Experiment
Results and discussion
Conclusions
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Introduction
• ZnO doped with group-III impurities (e.g., Al and Ga) has shown promising
results as a relatively inexpensive alternative TCO material with optical and
electrical properties comparable to those of ITO.2-5
• However, in the case of highly conducting TCOs, where extrinsic as well as
intrinsic donors are known to affect the carrier concentration and the
conductivity, the exact nature and interdependence of these entities is far from
understood.
• In this letter, we report the properties of highly degenerate Ga:ZnO thin films
and their dependence on vacancies, whose ionization efficiencies were found to
be dependent on extrinsic doping.
• In order to understand the fundamentals of carrier generation and transport
characteristics, we have investigated the temperature dependence of the
resistivity of Ga-doped ZnO thin films with varying vacancy concentrations.
• We present optical, structural, chemical, and electrical property measurements
to understand interesting characteristics of Ga:ZnO films.
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Experiment
• Zn0.95Ga0.05O films were grown on c-plane sapphire single crystal substrates by
pulsed-laser deposition PLD.9
• A pulsed KrF excimer laser with a wavelength of 248 nm was used for ablation.
The energy density of the laser beam was varied from 4–7 J /cm2 with a
repetition rate of 10 Hz.
• The chamber was evacuated to a base pressure of 1×10−3 Torr, and the
deposition was carried out at 2×10−2 Torr of oxygen pressure.
• The deposition was performed in the temperature range of 400–450°C for 12
min.
• X-ray diffraction XRD (θ-2θ scan) of the films was carried out using a Rigaku
X-ray diffractometer with Cu Kα radiation λ=1.54 Å and a Ni filter.
• A JEOL-2010 field emission transmission electron microscope TEM with an
attached gatan image filter was used to perform the structural characterization
of the film.
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• X-ray photoelectron spectroscopy XPS was performed using a Riber LAS3000 instrument with a Mg Kα x-ray source.
• Optical measurements absorption/ transmission were made using a Hitachi: U3010 Spectrophotometer, while the electrical resistivity was measured using
the four-point probe technique.
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Results and discussion
• Figure 1 shows an XRD trace for the
film grown at 400°C and 2.4×10−2
Torr of oxygen pressure, which
shows that the films are highly
textured along the c axis and aligned
with the (0006) peak of sapphire.
• The absence of additional peaks in
the XRD pattern excludes the
possibility of any extra phases and/or
large-size precipitates in the films.
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FIG. 1. XRD of the Zn0.95Ga0.05O film deposited at
400°C and 2×10−2 Torr of oxygen.
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• Film thickness was determined to be
240 nm, from a low magnification
cross-sectional TEM image shown in
Fig. 2(a).
• From Fig. 2(b), which is a HRTEM
image, it can be seen that the ZnO film
has grown epitaxially on sapphire and is
free of any nanosized clusters or
precipitates, which might have been
difficult to detect by XRD.
• SAED taken from the interface is shown
as an inset in Fig. 2(b), and illustrates
the following epitaxial relationship:
FIG. 2. (a) Low magnification image of the
film on sapphire. (b) HRTEM of the interface
and the inset showing the SAED of the same.
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• This epitaxial relationship corresponds to a 30° or 90° rotation in the basal plane
of the sapphire (0001) substrate. The epitaxy in such a large 16% misfit system
occurs as a result of domain matching epitaxy, where integral multiples of film
planes,
match with
of the substrate planes.11
• The absence of gallium oxide clusters, as observed from the HRTEM and XRD
analyses, indicates that Ga3+ occupies Zn substitutional sites and, therefore, can
act as an effective donor.
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• Figure 3(a) corresponds to the film grown
at 2×10−2 Torr of oxygen pressure, which
shows a MST; Metallic conductivity
above 170 K, and semiconducting
behavior at temperatures below it, as can
be clearly seen in the inset.
• Room-temperature resistivity of this film
was determined to be 1.4×10−4 Ωcm.
• Figure 3(b) is the plot of resistivity versus
temperature for the film grown in higher
oxygen pressure 1×10−1 Torr which has
the resistivity of 2×10−3Ωcm at ambient
temperatures.
FIG. 3. Plot of resistivity vs temperature for
(a) Zn0.95Ga0.05O film grown at 2×10−2 Torr of
oxygen and (b) 1×10−1 Torr of oxygen.
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• The resistivities of the films annealed at 600 °C for 15 min and undoped ZnO
were found to be on the order of ~10−1 Ωcm.
• The metallic conductivity observed here can be explained by the formation of a
degenerate band appearing in heavily doped semiconductors as suggested by
Mott.8
• The increase in conductivity of ZnO thin films is a combined effect of Ga
doping and oxygen vacancies, which act as donors and lead to degeneracy.16,17
• However, the increase in resistivity—observed for annealed samples and for
those grown in higher oxygen pressure—suggests that the oxygen vacancies
are the major contributors to the increase in carrier concentration.
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• The transmittance of all the films in visible
region was found to be ~80%. It can also be
seen from the figure that for the film with
the lowest resistivity, the absorption edge is
shifted toward higher energy due to the
Burstein–Moss effect.18,19
• However, on annealing, the absorption edge
moves to higher wavelength, indicating the
decrease in carrier concentration due to
reduction in the number of oxygen
vacancies.17,20
• Photoluminescence measurements also
showed similar peak shifts, further
strengthening our argument that the carrier
concentration is increased partly due to the
increased number of vacancies.
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FIG. 4. Transmission spectra of (a) as-deposited
Zn0.95Ga0.05O film grown at 2×10−2 Torr of
oxygen, (b) same film annealed in air at 600°C
for 15 min, (c) as-deposited Zn0.95Ga0.05O
film grown at 1×10−1 Torr of oxygen, and (d) asdeposited ZnO film grown at 2×10−2 Torr.
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• Although, vacancies play an important role in lowering the resistivities in Gadoped ZnO, they are not as effective in pure ZnO, since pure ZnO grown under
a similar growth condition showed considerably higher resistivity and also
exhibited semiconducting behavior.
• This indicates that the energy levels of oxygen vacancies in ZnO are effected to
a certain extent by dopants, such as gallium.
• Thus, gallium not only acts as an effective donor, but also changes the oxygen
vacancy characteristics, which leads to very low resistivities.
• The presence of impurity atoms, such as gallium, can lead to disorder in a
degenerate semiconductor, resulting in localization of the electronic states21
and corresponding increase in resistivity with a negative TCR, as observed in
Fig. 4(a).
• However, at temperatures higher than the transition, electrons are delocalized
due to thermal activation, and the conductivity is dominated by phonon
scattering. Further work is needed in order to fully understand the effect of
gallium on electrical properties of ZnO, especially at low temperatures.
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Conclusions
• 本研究利用PLD成功製備出ZnO:Ga(5%)薄膜，結合最佳之摻雜與氧空缺
濃度，使其具有低電阻率(1.5×10−4 Ωcm )與高穿透率(~80%)之薄膜。
• 在GZO薄膜的電阻率中，也觀察到金屬半導體傳輸(MST)特性的金屬導電
特性。
• 在XRD和HRTEM中證實了薄膜的磊晶性質(the epitaxial nature of the films)，
同時也得知在薄膜中並無任何的晶相、團簇與析出物的產生。
• 從XPS分析得知，摻雜進GZO薄膜的鎵以Ga3+的形式存在，並扮演著有效
施體的角色。
• 薄膜高導電率主要來自氧空缺與摻雜物的交互作用，氧空缺的增加等同
於載子濃度的增加，但氧空缺的電子能階或電離效率則是受到摻雜物的
影響而改變。
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Thanks for your attention
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